This invention relates to apparatus and methods for measuring strain produced by forces acting on a body or member. More particularly, it relates to such apparatus and methods capable of utilizing individually conventional strain gage elements, for example to enable measurement of a force to which the body or member is subjected. In a still more particular sense, it concerns the making of such measurements in a member which is axially elongated and generally radially symmetrical in transverse cross-section, viz. a member having the general configuration of a rod or shaft, such member being herein termed a pin.
A typical strain gage element includes an array of runs of wire electrically connected in a suitable circuit (ordinarily, a Wheatstone bridge circuit) and mechanically connected to a body or member in a predetermined orientation relative thereto such that strain applied to the body or member stresses (and thereby alters the electrical resistance of) the wire. Frequently a plurality of strain gage elements in a common bridge circuit are mechanically connected to the body or member, with respectively different orientations. Since the variation in resistance of a strain gage element is proportional to the applied strain, the voltage output of the bridge circuit can be read as a measure of the forces (acting on the body or member) that produce the detected strain. Shear, torsion, bending, tension and compression strains can all be selectively measured in this way, depending on the location and orientation of the strain gage element or elements relative to the body or member. Methods and apparatus employing strain gage elements for these purposes are described, for example, in U.S. Pat. No. 4,530,245.
In one important particular aspect, to which detailed reference will be made herein by way of specific illustration, the present invention is directed to the measurement of shear strain in a shear pin, e.g. as employed in a tongue-and-clevis coupling between a tractor and a plough. Measurement of drag force on a tractor, by detection of shear strain on the pin, has recognized utility for control of ploughing operations. Heretofore it has been known to provide a shear pin with an axial bore extending at least part way along the length of the pin, and to bond strain gage elements to the inner wall of the bore at a location at which they will sense the strain produced by shear when the pin is subjected to forces resulting in shear. The difficulty of properly positioning and mounting the strain gage elements within the bore, however, precludes mass production of shear pins equipped with strain gage elements and causes such pins to be undesirably high in cost.